1. Field of the Invention
The present invention relates to vehicle brake systems and more particularly to an improved electric power booster.
2. Description of the Related Art
Many of today's vehicles are equipped with power-braking systems to reduce a driver's brake pedal effort. Most use a vacuum to increase braking power. An engine's pistons create a vacuum as they draw air into the engine. The engine intake manifold vacuum is connected by a tube to both sides of a special spring-loaded diaphragm located in a housing intermediate an operator brake actuating pedal mechanism and a master cylinder. When the operator actuates the brake pedal, atmospheric pressure is admitted to one side of the diaphragm supplying additional force to the master cylinder piston and enhanced brake line pressure to the individual wheel cylinders. Vehicle braking is still possible in the event of vacuum source failure since brake pedal pressure may still be directly applied to the master cylinder piston. Vacuum boosters of this type are illustrated in numerous patents, for example, U.S. Pat. No. 5,943,863.
Many vehicles, for example, hybrid vehicles or those employing diesel engines, lack a vacuum source and may utilize a power steering pump or a separate dedicated pump as a hydraulic pressure source, or may employ a separate air compressor for air brakes. All of these systems rely on operation of the vehicle engine to augment operator brake pedal input force.
Several systems eliminate the dependence on manifold vacuum or other power source dependent on engine operation by substituting an electromechanical mechanism for above described vacuum booster. Electric brakes employing electromagnets at individual wheel brake locations to force braking surfaces together have been suggested.
In U.S. Pat. No. 4,395,883, brake pedal force is amplified by an electric motor acting through a gear set which converts, by way of a ball screw arrangement, rotary motion and torque from the motor into linear motion and force to energize the master cylinder piston. Operator input push rod forces are sensed, amplified and modulated by electronic means. The electric motor disengages from the gear set at rest or if there is an electrical failure, so that there is a manual mode of operation available when necessary. A solenoid is used to operate a power clutch to separate the power and no-power apply modes. Suitable controls are provided by force sensing means such as a piezoelectric crystal or some other suitable pressure transducer, and suitable circuitry including control logic in the nature of a microprocessor.
U.S. Pat. Nos. 6,574,959 B2 and 6,758,041 B2 disclose similar systems. The U.S. Pat. No. 6,574,959 arrangement employs a transmission system including a belt wrapped around pulleys to transfer power from a motor to a ball screw, and suggests belts, toothed belts, chain belts or gears and that the pulleys can be sprockets or the like. In the U.S. Pat. No. 6,758,041, a ball screw assembly is operatively connected by pinion and a ball screw drive gears between the output shaft of the electric motor and the booster shaft for converting torque from the motor into axial force applied to the booster shaft. Brake pedal input force is measured by a piezoelectric or magneto-restrictive sensor.
Each of these patented electric booster arrangements is disposed intermediate the brake pedal push rod and the master cylinder input piston rod just as in the case of the traditional vacuum booster. Each employs a rather complex mechanical coupling between the motor and master cylinder piston rod.
It is desirable in an electric booster to simplify the interface between the motor/driveline mechanical components and the hydraulic pistons that transmit fluid to each respective brake circuit.